The US forces will continue to train and advise Afghan forces, and will focus on ensuring that any remnants of Al-Qaeda are prevented from posing a threat to US security, senior administration officials told Reuters.

NATO Secretary General Jens Stoltenberg said that Obama’s decision "paves the way for a sustained presence by the NATO alliance".

"In the coming weeks the alliance will take key decisions on the future scope of the rest of its support mission," he said. "The significant U.S. contribution will be an important factor as we consider the way ahead."

The decision is a deviation from the original plan to pull out all but a small, embassy-based US military presence by the end of next year.

It comes after military leaders argued that Afghans need additional assistance and support from the US in order to beat back the Taliban and hold onto gains made over the past 14 years.

"The narrative that we're leaving Afghanistan is self-defeating,'" Defense Secretary Ash Carter said Wednesday during a speech at the Association of the US Army. "We're not, we can't and to do so would not be to take advantage of the success we've had to date.''

The cost of keeping 5,500 troops in the country will cost an estimated $14.6 billion per year, up from the predicted $10 billion to keep a consolidated force at the US embassy, an official said.

Afghan forces have struggled following recent asssaults from Taliban militants, who briefly captured the city of Kunduz and other territories, in what was deemed its biggest victory in 15 years.

Earlier this month, US forces admitted to conducting an airstrike which killed 22 people at a hospital in Kunduz. Obama apologized for the attack, saying the hospital was "mistakenly struck."

Russia's Foreign Ministry said on Thursday that it doubts that Washington’s decision to maintain the number of troops in Afghanistan will ease the situation in the country, adding that the West should offer Afghanistan “real help.”

“I don’t know what it’s going to change. I have stated earlier that if 100,000 [troops] did not succeed, what can 5,000-6,000 do?” said Zamir Kabulov, director of the Russian Foreign Ministry's Second Department.